A vinyl ether polymer having a hydroxyl group on the side chain is useful as a water-soluble polymer or a temperature-responsive polymer.
For example, homopolymers of 2-hydroxyethyl vinyl ether (HEVE) and diethylene glycol monovinyl ether (DEGV) are known to be water-soluble and homopolymers of 4-hydroxybutyl vinyl ether (HBVE), HBVE/HEVE copolymers, and HBEV/DEGV copolymers are known to be temperature-responsive polymers that have variable hydrophilicity/hydrophobicity depending on the temperature.
Since generally vinyl ether is cation-polymerizable, radical polymerization is difficult to carry out with vinyl ether alone but still possible to carry out with the sole use of vinyl ether having a hydroxyl group; for example, radical polymerization can be performed by using an oil-soluble polymerization initiator in the presence of an inorganic solvent or a water-soluble organic solvent such as methanol (Patent Document 1). However, the conversion rate of the monomers was low and vinyl ether was not efficiently radical-polymerized in said method.
It was reported that when a water-soluble polymerization initiator was used, more polyacetals were generated than the target vinyl ether polymers (same Patent Document 1).
Meanwhile, the vinyl ether polymer having a hydroxyl group on the side chain is generally a highly viscous liquid polymer and has a problem of poor workability and processability. In addition, although these vinyl ether polymers are often used as a solution or an aqueous composition, the application is limited since it is difficult to use in high concentration due to its high viscosity.
The method to reduce the viscosity of the polymer includes, for example, a method of using a polymer having a smaller molecular weight. A method of using a chain transfer agent (a molecular weight modifier) is known as the method for reducing the molecular weight of the polymer in the radical polymerization.
For example, Patent Document 2 discloses a method for producing a low molecular weight polyacrylic acid by using secondary alcohols such as propylene glycol, 2-hexanol, and the like as the chain transfer agent. Patent Document 3 discloses a method for producing a low molecular weight soluble acrylic resin by radical polymerization in a solvent having a specific chain transfer constant such as isopropanol or in a mixed solvent of those and water.
However, in the case of conducting radical polymerization of vinyl ether having a hydroxyl group only, there has been no case in which a chain transfer agent was used or where the chain transfer constant was considered. Since the chain transfer agent greatly changes its performance depending on the type of monomers, it is not possible to obtain a suitable chain transfer agent or attain the knowledge of controlling molecular weights in the radical polymerization of vinyl ether having a hydroxyl group from the radical polymerization of an acrylic monomer as described in said Patent Document 2.
The temperature-responsive polymer generally stops showing a sharp temperature-responsiveness when the molecular weight of the polymer, the homogeneity of the composition ratio and the like are low. In the case where a chain transfer agent is used, a polymer having a terminal structure derived from the chain transfer agent and a polymer having a terminal structure derived from the initiator will be present in a mixed state, thereby making it difficult to predict the effect of the mixed presence of the terminal structure derived from the chain transfer agent on the temperature-responsiveness of the polymer.
Further, in the case of the vinyl ether polymer having a hydroxyl group on the side chain, it is considered not possible to obtain a sufficient effect of reduction in viscosity just by reducing the molecular weight since the effect is large of, not only the intertangled molecular chains, but also the hydrogen bonds by the side chain hydroxyl groups.
As for reducing the hydrogen bonds by the side chain hydroxyl groups in the case of the vinyl ether polymer having a hydroxyl group on the side chain, there can be considered a method of introducing vinyl ether which has no hydroxyl group such as alkyl vinyl ether as a hydrophobic unit.
However, since vinyl ether which has no hydroxyl group is low in reactivity by radical polymerization, it is difficult to introduce the hydrophobic units quantitatively by the radical polymerization. Accordingly, the obtained polymers result in inhomogeneous molecular weights and composition ratios, which give rise to the problem of impaired performance such as solubility to water, temperature-responsiveness, and the like.
In order to synthesize polymers having homogeneous molecular weights, composition ratios, and the like by co-polymerization of vinyl ether having a hydroxyl group and vinyl ether without a hydroxyl group, a method is known in which living cationic polymerization is performed by using a monomer by protecting a hydroxyl group with a protective reagent (a capping agent) and deprotection of the protective group is performed after polymerization.
However, the living cationic polymerization is normally performed under a low temperature of 0° C. or less and temperature control is not easy in an industrial scale due to the reaction heat and in addition, the reaction needs to be performed under an anhydrous state and in inert gas (Non Patent Document 1). Therefore, it is difficult to attain production in industrially low cost and of efficiency. Also, the method in which the monomer having a hydroxyl group subjected to capping is used requires a deprotection step of the capping agent after polymerization, which the deprotection is generally carried out with the use of acid or alkaline that needs to be removed in a removal step. Such increase in the number of steps will directly lead to the increase of manufacturing cost of the polymers.
As for the method for introducing the hydrophobic units, there can be considered, apart from those described above, for example, a method in which the side chain hydroxyl group after polymerization is partially reacted with the capping agent.
However, such method will require a capping step of the hydroxyl group, as well as the polymerization step, which is a drawback that increases the manufacturing cost of the polymers. Also, it is difficult to control the protection rate in the partial capping and to produce polymers having the same quality with good reproducibility in industrial terms and further, there is a concern that the protection rate changes due to, for example, hydrolysis of the capping agent depending on the storage conditions to affect the performance of, for example, solubility to water, temperature-responsiveness, and the like.
In the method of introducing the hydrophobic units, the hydrophobic unit itself is a disincentive of the solubility to water, the temperature-responsiveness, and the like and especially the low molecular weight compound is greatly susceptible to the hydrophobic unit in both methods; therefore, there is a limit in the method for suppressing the viscosity elevation due to the hydrogen bond by introducing the hydrophobic unit.